Extractive distillation process



De@ 7, 1948- G. J. PIRo'rTl y 2,455,803

EXTRACTIVE `DISTILIJVIATION PROCESS Filed Feb. 1l, 1944 Invenfor: GinoJ'. Pieroti lfatented Dec. 7, 1948 EXTRACTIVE DISTILLATION PROCESS GinoJ. Pierotti, Albany, Calif., assignor to Shell Development Company, SanFrancisco, Calif.,

a corporation of Delaware vApplication February 11, 1944, serial No.522,206 Claims. (Cl. 2oz-39.5)

This invention relates to a process for separating vaporizable organicmixtures by extractive distillation with a solvent comprising (1) aselective solvent and (2) a mutual solvent for said selective solventa'nd said mixture. This application is a continuation-impart of mycopending application, Serial No. 475,213 filed February 8, i943 nowabandoned.

Extractive distillation calls for the employment of a vapor phaseextractor comprising in a single column from top to bottom, a solventknock-back zone, a rectifying zone, and a stripping.v zone, the latterbeing equipped with a reboiler (see drawing). During operation, a liquidselective solvent flows downward throughthe rectifylng and strippingvones in countercurrent to the mixture to be separated. Naturally, it isdesirable to employ a solvent of high selectivity. However, highselectivity in a solvent is frequently coupled with low solvent power`and if its power is low it tends to form two liquid layers in theextractor. This-I have found-results in poor extraction eiliciency.

it is the broad purpose of this invention to maintain a single liquidphase between a. selective solvent of low solvent power and the mixtureto be separated in an extractive distillation column. Another purpose ofthis invention is to separate vaporizable organic mixtures economicaily,eillciently, and eiectively by extractive distillation using a selectivesolvent of low solvent power. Speciiic purposes, among others, are toproduce pure compounds, to desulfurize hydro.

carbon mixtures, to dehydrate wet compounds, to separate components ofconstant boiling mixtures, or of mixtures having boiling points closerthan say C., or isomeric mixtures, or other mixtures of similar organiccompounds having diierent degrees of saturation, or other mixturesotherwise diflicultly separable. More speciilcally, a purpose of thisinvention is to produce pure compounds such as butadiene, toluene,xylenes, etc. from hydrocarbon mixtures containing them.

Generally, the process of this invention comprises the following steps:(A) Countercurrently contacting the mixture to be separated in the vaporstate, that is, at a temperature above its bubble temperature, in arectifying zone with a high boiling liquid solventk comprising a highlyselective solvent and `a mutual solvent, to pro- 2 duce a raillnatevapor which is removed, and a rich solvent. (B) The latter is thenstripped in a stripping zone comprising a reboiler or similar piece oi'equipment, to drive out dissolved components of the mixture having thelower solubility in said solvent. The stripped rich solvent is theextract phase which consists essentially oi.'

the highly selective solvent, the mutual solvent, and the components ofthe mixture of relatively high solubility. Step (C) comprises separatingthe extract components from said selective solvent and mutual solvent bydistillation or washing out. An optional step (D) comprises returningthe two solvents to the extractor for further contact with more of saidmixture. Additional steps of pre-fractionation of the mixture to beseparated and of fractional distillation of the final product areusually employed in the production of pure products.

Before discussing the process in detail, the mixtures and solventsinvolved in the process will be more fully described.

Tm: Mummia 'ro as Seminaren Many diierent types of mixtures of compoundsmay be separated by the extractive distillation, provided the mixturesare at least partially vaporizable at temperatures substantially belowthe boiling temperatures of the solvents and are inert toward them.Moreover, the presence of the solvents in the mixture must cause agreater change in the escaping tendency of one component of the mixturerelative to that of the other components. By escaping tendency is meantthe potential of one component to pass from.. one phaseto another. Theprocess may be used for the isolation of pure compounds, the separationof isomers, various purification processes such as desulfurization anddehydration, the separation of mixtures forming azeotropes or theseparation oi mixtures of organic compounds of different degrees ofsaturation, for instance the concentration of the diilerent types ofhy.. drocarbons in diierent fractions, examples being the separation ofaromatics, acetylenes, poly oleins, olens, naphthenes and paramns fromeachother in various hydrocarbon mixtures con-l taining them.

' -YSome specic examples of mixtures which may be separated into two ormore components by cameos the selective solvents of this invention arenarrow boiling mixtures containing two or more of the followinghydrocarbons having the same number of carbon atoms but differing in thedegrees of saturation: ethane, ethylene, acetylene, propane,.'propylene, allylene, methyl acetylene, normal and isobutane, normal,iso, alpha, and beta-butylenes, butadienes, dimethyl acetylene, ethylacetylene, vinyl acetylene, diacetylene, etc.; normal and isopentanes,normal and iso amylenes, isoprene, piperylene, n-propyl acetylene,isopropyl acetylene, methyl ethyl acetylene, cyclopentanes,cyclopentenes, and cyclopentadienes, etc.; normal and isohexanes, normaland iso hexylenes, l'iexadiene-L, methyl pentadienel,3, isobutylacetylenes, diethyl acetylenes, di-propargyl, dimethyl di-acetylene,methyl cyclopentane, cyclohexane, cyclohexadiene, etc.; normal and isoheptanes, normal and iso heptylenes, etc.; normal and iso octanes,normal and iso-octylenes. di-isobutenyl, etc.; gasoline distillatescontaining benzene, toluene, xylenes, ethyl benzene, mesitylene, cumene,etc. togther with naphthenes or parafllns or both; chlorinatedhydrocarbon mixtures, etc. Other mixtures which may be separated arethose of organic substances containing water, such as aqueous alcoholsincluding methyl, ethyl, propyl, etc. alcohols; or organic acidsincluding formic, acetic propionic, etc. acids; or esters includingmethyl, ethyl, isopropyl, formate or acetate, etc. Still other mixturesare those of oxy organic compounds such as primary and secondary butylalcohols; methyl propyl ketone and di-ethyl ketone; etc. Other organicmixtures are those produced in various industrial and chemical processesof the coal, lignite, and petroleum industries such as organic sulfurcompounds, including mercaptans, mixtures of phenols and thio phenols,mono, di, and tri-methyl amines; isoprene and methyl formate; etc'.

It may be noted that all of the above mixtures are at least partiallysoluble in the commonly known selective solvents such as furfural,phenol, etc. which have preferential solvent power for aromatic overparafnnic hydrocarbons.

THE SELrzc'rrvs SoLvEN'r The selective solvent should boil at at least50 C. and preferably at more than 100 C. above the A. S. T. M. 90%boiling temperature and preferably above the end boiling point of themixture to be separated. It must be sufficiently heat stable to at leastpartially vaporize without decomposition in the presence of theauxiliary solvent. Moreover, it should not crystallize out of solutionat the highest concentration which may occur at any point in theprocess. It is desirable that it be not more than 50% by volume solublein a kerosene having a Watson Characterization Factor of at least 12(see Ind. and Eng. Chem., vol 27, p. 1460, December, 1935,Characterization of Petroleum Fractions by Watson, Nelson and Murphy).Higher solubility in such a kerone frequently is an indication of poorselectiv- It is understood that certain of the selective solventsproduce two liquid phases in an extractive distillation column with somemixtures but not with others. Whether or not two liquid phases areformed can easily be tested in the laboratory.

It is understood that different mixtures may require diiferent solvents.By way of example,

viz. sulfolane, methyl suliolane, sulfolanol, di-

methyl sulfone, ethylene glycol, diethylene glycol,

'.higher poly ethylene glycols, glycerne, certain some specic solventswhich generally form two liquid phases with hydrocarbons are listedbelow,

glyceryl mono ethers and esters, such as methyl, ethyl, propyl, butyl,amyl, hexyl, cyclohexyl, phenyl, tolyl, xylyl, decyl, hexadecyl,octadecyl, etc. glyceryl esters and ethers; dihydroxy diphenyl ether,glycerol chlorhydrin, mono, di and tri ethanol amines, amino diols suchas amino propylene and amino butylenev glycol, di-amino glycols such asdi-amino propanol, di-amino butanol, etc.; and many others.

All of the above-mentioned selective solvents are miscible with water inall proportions, but not all highly sensitive solvents which aremiscible with water form two liquid phases with hydrocarbons inextractive distillation.

THE MUTUAL SoLvEN'r The selection of the mutual solvent depends upon thetype of mixture to be separated and the selective solvent employed, inthat its purpose is to produce a single liquid phase between them attheir bubble temperature or under the conditions existing in anextractive distillation column. The boiling temperature of the mutualsolvent mustbe suiliciently above the A. S. T. M. boiling point of themixture, so that it may be easily separated from the bulk of the mixtureby ordinary fractional distillation and remain with the highly selectivesolvent. Thus it is desirable that the mutual solvent have a boilingpoint not less than 10 C. and preferably more than 40 C. higher than theA. S. T. M. 90% boiling temperature and preferably above the end boilingpoint of the mixture to be separated. In addition, the mutual solventmay decrease the boiling temperature oi the highly selective solvent,thereby reducing the temperature in the extraction column and the dangerof thermal decomposition of the highly selective solvent. The mutualsolvent may chemically resemble very closely the component of themixture extracted by the selective solvent, provided it meets the aboverequirements. In effect, the mutual solvent may be introduced as part ofthe mixture to be separated.

Some suitable mutual solvents which may be employed in the separation ofhydrocarbon mixtures in conjunction with the highly selective sol'-vents mentioned above are mono ketones, alcohols, and ethers containingfrom about 3 to 8 carbon atoms per molecule such as methyl ketone,diethyl ketone, methyl isobutyl ketone, diisobutyl ketone,cyclohexanone, n or isopropyl, butyl, amyl, hexyl, cyclohexyl, etc.alcohols, di'- ethyl, dipropyl, methyl propyl, methyl butyl, etc.ethers; acetonitrile, lactonitrile, etc.; dioxane, pyridine, methylpyridine, morpholine, aromatic hydrocarbons such as benzene, toluene,cumene,y mesitylene, etc. and others.

As in the case of the selective solvents, some `of these solvents mayact as mutual solvents for some combinations and not for others. It isunderstood that the mutual solvent selected will have previously beentested in combination with the selective solvent and the mixture to beseparated in order to determine its effectivness to insure a singleliquid phase throughout the conditions existing in the extractivedistillation.

THE PROCESS Referring to Figure l of the drawing, the feed f mixture tobe separated is introduced into column I through line 2 where itcountercurrently contacts a solution of a selective solvent and a mutualsolvent introduced through line 3, at a temperature slightly above thebubble temperature of said mixture, to produce a ralnate vapor and aliquid extract phase. Column I may be a packed or bubble platefractionating column and is provided at its bottom with reboiler 4 tovaporize the mixture and solvents, and at its top with a vapor line 5,condenser 6 and accumulator 'I to provide a reflux of the raiiinate tothe top of the column through valved line 8, and a ratllnate productwithdrawn through valved line 9. Column I has three zones. a solventknockback zone above the solvent inlet 3, a rectifying zone betweeninlet 3 and feed line 2, and a stripping zone below feed line 2.

The extract phase, comprising the selective solvent, the mutual solvent.and the dissolved component of the mixture, is withdrawn from column Ithrough bottom line I and is introduced into stripper II to remove saidextract from the solvents. Stripper II is provided with reboiler I2 atits bottom, top vapor line I3, condenser I4, and accumulator I toprovide a reflux of extract to the column through 'valved line I6 and anextract product withdrawn through valved line I1.

Recovered solvents are withdrawn from stripper I I through bottom line I0 to join line 3 for recontact with more of said mixture in saidextractor I. Valved line I9 is provided for the addition of freshsolvents to the system as required.

'I'his extractive distillation process may be carried out eitheradiabatically or isothermally.

The apparatus employed in this process may be in any conventional orconvenient type known to those skilled in the art. The temperature forextractive distillation with a highly selective solform under theconditions o! the lprocess or at the bubble temperature ot the mixtureto be sepa rated. It preferably should not be greater than about 1:1 byvolume. because excessive quantities Example I A mixture of toluene(boiling point 110.6 C.) and paraiiins having a, boiling range close tothe boiling point of toluene was admixed in a vessel with 50% by weightof each of the selective solvents shown in the table below, both withand without mutual solvents. Each mixture was then.

heated at thev boiling point until the vapor above the liquid was inequilibrium with the liquid. 'I'hen samples of vapor were separated,condensed, and tested to determine the percent of toluene in the vaporon basis free from all solvents. From these data the volatility ratio oftoluene to paraffins in the presence of each solvent was calculated. Thevolatility ratio, often called the alpha value, the vapor to the percentof parains in the liquid, divided by the ratio of the percent of toluenein the vapor to the percent of toluene in the liquid. In comparison, thevolatility ratio of toulene to paraflin in the mixture without theaddition of a solvent is also shown. The alpha value is a direct measureof the selectivity of the solvent, and the greater this value thegreater is the selectivity.

In the table below the essential data are summarized and for comparisonthe alpha values of two Widely used common selective solvents are ventand a mutual solvent generally may range 40 also shown.

. Wt. Per Cent N t.

Wt. Per Wt. Per Toluene in- A] ha I ioijd Selective Solvent Cent onMutual Solvent Cent on Vaup hqw g Whole Mix Whole Mix t B Liquid Vapor aNone 50. 0 43.1 1. 32 1 Methyl-S-sulfolane 50. 0 32. 6 2. 07 2 50 50. 029. 9 2. 58 l 5() 50. 0 26. 6 2. 76 l 50 50. 0 26. 8 2. 72 l 50 50. 037. 0 1. 70 2 50 50. 0 23. 2 3. 30 l 50 60. 0 36. 7 l. 73 2 50 50. 0 27.6 2. nl 1 50 50. 0 61. 2 l. 58 2 26 50. 0 70. 3 2. 43 l 50 50. 0 64. 9l. 85 1 24 50. 0 60. 6 1. 54 l 50 50. I] 32. 3 2. l0 l 50 60. 0 32. 5 2.O8 l within wide limits provided it is above the bubble temperature ofthe mixture and substantially below the boiling temperature of thesolvents under the pressure and temperature conditions maintained in theextractor. Sub-atmospheric pressures may be resorted to in order toreduce the temperature if the mixture is not thermally stable at highertemperatures. Suitable temperatures may range from above about -50 C. orthe bubble temperature of the mixture (whichever is higher) up to about+350 C. at pressures from about .1 p. s, l. absolute up to about 500 p.s. i. absolute.

The ratio ofv mutual solvent to highly selective solvent must be atleast enough so that through- Example II A mixture of equal weights ofisoprene (boiling point 34 C.) and pentene-2 (boiling point 36.4 C.) wasadmixed in a vessel with diierent amounts by weight of each of theselective solvents shown in the table below, both with and withoutmutual solvents. Each mixture was then heated to its boiling point untilthe vapor above the liquid was in equilibrium with the liquid and thensamples of the vapor were separated, condensed, and tested to determinethe percent of isoprene in the vapor on a, solvent-free basis. Fromthese data, the volatility ratio of isoprene to pentene2 in the presenceof each solvent iS out the contact zone only one liquid phase will 75calculated.

is the ratio of the percent of paraiiins in aeeasca in the table belowthe essential data are summ:

two liquid phases unde the conditions of the process, the improvementcomprising adding Wt. per cent oi Wt. per cent Wt. per cent Pantone-2 inAl ha No. oi Selective Solvent in final Mutual Solvent in iinnl V319liquid mixture mixture ue phases Liquid Vapor 50.0 49.0 0.98 l 86 No 50.0 1 1. 15 2 64 Acetonitrile li 50. 0 2 72 do 13 50.0 67.9 1.56 1 52Dioxane.. 23 50. 0 66. 2 1. 48 1 85 no 0 50. 0 50. 7 1. 03 2 88Acetonltrila 17 50. 0 2 38 -.do. 37 50.0 67.8 1.55 1 51 Di0xane 34 50. 067. l 1. 52 1 68 Acetone i7 50. 0 67. 1 l. 52 1 72 Methyl ethyl ketone.13 50. 0 2 68 .do 17 50.0 69.4 1.63 1 68 Methyl isobutyl ketone. 17 50.0 67. 9 l. 56 1 a isopropyl alcohol 25 50. 0 67.1 1. 52 l 72 pyridine iaso. o 2 00 do 25 50.0 68.7 1.60 1 68 Secondary butyl alcohol 17 50. 070. 6 1. 70 l l Estimated value within =i=.l oi correct "Alpha Value.

I claim as my invention:

1. In an extractive distillation process for the separation of avaporizable organic mixture comprising predominantly aromatichydrocarbons and non-aromatic hydrocarbons of similar boiling range witha sulfolane selective solvent there'- i'or, wherein said selectivesolvent forms two liquid phases under the conditions of the process, theimprovement comprising adding to said sulfolane solvent from anextraneous source a mutual solvent which is an aromatic hydrocarbon forsaid selective solvent and said mixture in amounts corresponding to amutual solvent-tosulfolane volume ratio not greater than about unity andsuiilcient to produce a single liquid phase under the conditions of theprocess, whereby the emciency of the extractive distillation isimproved, both said solvents having boiling temperatures above the A.`S. T. M. 90% boiling temperature of said organic mixture andsufficiently higher whereby separation of the solvents together from theresulting extracted portion of the mixture may be readily eiiected bydistillation.

2. In an extractive distillation process for the separation of anorganic mixture comprising predominantly toluene and parafilnichydrocarbons having a boiling range close to the boiling point oftoluene with a sulfolane selective solvent therefor. wherein saidselective solvent forms two liquid phases under tbe conditions of theprocess, the improvement comprising adding to said sulfolane solventfrom an extraneous source a mutual solvent which is an aromatichydrocarbon for said selective solvent and said mixture in amountscorresponding to a mutual solvent-to-l sulfolane volume ratio notgreater than about unity and suilicient to produce a single liquid phaseunder the conditions of the process. whereby the eiiiciency of theextractive distillation is improved. both said solvents having boilingtemperatures above the A. S. T. M. 90% boiling temperature of saidorganic mixture and suiliciently higher whereby separation of thesolvents together from the resulting extracted portion ot the mixturemay be readily effected by distillation.

3. In an extractive distillation process for the separation of anorganic mixture comprising predominantly toluene and non-aromatichydrocarbons having a boiling range close to the boiling point oftoluene with a sulfolane selective solvent therefor, wherein saidselective solvent forms cumene to said sulfolane solvent from anextraneous source as a mutual solvent for said selective solvent andsaid mixture in amounts corresponding to a mutual solvent-to-sulfolanevolume ratio not greater than about unity and suillcient to produce asingle liquid phase under the conditions of the process, whereby theeiciency of the extractive distillation is improved.

4. In an extractive distillation process for the separation of anorganic mixture comprising predominantly toluene and non-aromatichydrocarbons having a boiling range close to the boiling point oftoluene with a sulfolane selective solvent therefor, wherein saidselective solvent forms two liquid phases under the conditions of theprocess, the improvement comprising adding mesitylene to said sulfolanesolvent from an extraneous source as a mutual solvent for said selectivesolvent and said mixture in amounts corresponding to a mutualsolvent-to-sulfolane volume ratio not greater than about unity andsufiicient to produce a single liquid phase under the conditions of theprocess, whereby the emciency of the extractive distillation isimproved.

5. In an extractive distillation process for the separation of anorganic mixture comprising predominantly toluene and non-aromatichydrocarbons having a boiling range close to the boiling point oftoluene with suliolane under conditions which normally form two liquidphases, the improvement comprising adding cumene to said sulfolane froman extraneous source as a mutual solvent for said sulfolane and saidmixture in amounts corresponding to a mutual solventwsulfolane volumeratio not greater than about unity and suillcient to produce a singleliquid phase under the conditions of the process, whereby the emciencyof the extractive distillation is improved.

GINO J. PIERO'I'II.

REFERENCES CITED The following references are of record in the ille ofthis patent:

UNITED STATES PATENTS Number Name Date 1,474,216 Van Ruymbeke Nov. 13,1923 2,162,963 McKittrick June 20, i939 2,290,636 Deanesly July 21, 19422,325,329 Durrum July 27, 1943 2,350,584 Buell June 6, 1944 (Otherreferences on following page) UNITED STATES PATENTS Number Name Date2,357,028 Sluras Aug. 29, 1944 2,360,859 Evans Oct. 24, 1944 5 2,360,861Pierotti et al. Oct. 24, 1944 2,366,360 Semon Jan. 2, 1945 2,396,302Cummings et al. Mar, l2, 1946 2,434,796 Hackmuth Jan. 20, 1948 OTHERREFERENCESA Moor et a1., Extraction of 1,3 Butadne from Cracked Gases.II, Comparative Solubilities of

